The present invention relates generally to inspection devices, and more particularly to a device which can perform inspections of substrates and cover glass on a substrate using a dark-field imaging technique.
Cracks in a substrate or in the cover glass over a substrate can result in substrate failures or failures in the system in which the substrate is placed. In particular, when the substrate is a solar cell, a crack in either the solar cell or the cover glass over the solar cell has the potential to severely limit the power output of the solar panel which contains the crack. Whereas a minor crack in the cover glass can usually be repaired, for example by patching with sealants or adhesives, solar cell cracks cannot usually be repaired. In most cases, the solar cell having a crack is removed and is replaced with a new solar cell. It is therefore important to detect cracks in both the substrate and the cover glass and to differentiate cracks in the solar cell from cracks in the cover glass, especially if the crack in the cover glass occur in approximately the same vicinity as a crack in the solar cell thereby obscuring the crack in the solar cell. The current techniques for inspection of solar cells cannot reliably detect both cell cracks and glass cracks simultaneously, nor the coincident cracks.
Current techniques used to detect cracks in cover glass on a substrate such as a solar cell involves a visual inspection. An inspector illuminates the cover glass and the solar cell with properly controlled lighting and physically examines the cover glass with a magnifier looking for defects and cracks. This technique requires the inspector to turn his head around at many different angles to thoroughly inspect the substrate. This physical inspection technique is a time-consuming procedure which can cause fatigue of the inspector's eyes and defects being overlooked due to human error.
What is needed therefore is an apparatus and method for detecting cracks in a substrate and in particular, cracks in a solar cell and the cover glass over the solar cell, and also differentiate between cracks in a solar cell and cracks in the cover glass over the solar cell which provides consistent detection and differentiation of cracks but does not require manual inspection.